Because in recent years US politics has become so divided that it has proved impossible to pass a budget through Congress. Instead, a stop-gap funding measure called a “Continuing Resolution” or “CR” for short, has been passed to keep the government funded. The current CR runs out at midnight on Monday and as yet there is no agreement to pass a new one.

Just checking, but is the CR the same as the debt ceiling?

No. The Debt Ceiling is an entirely different and potentially far more serious issue. This is the upper limit of money the US government is allowed to borrow. It is set by Congress. The US treasury estimates that the current ceiling will be exhausted on October 17.

The current ‘continuing resolution’ sets US government funding at $984bn (£609bn). The debt ceiling is currently set at $16.699 trillion (£10.35 tr). The debt ceiling was first set in 1917 and has been raised 79 times since 1940.

Okay. So why won’t Congress pass a new CR, or vote to raise the debt ceiling?

Because a small rump of highly ideological, small-government Republicans are refusing to do either unless they get some major policy concessions from the Obama administration – most notably defunding or delaying Obamacare, the signature healthcare reforms of the president’s first term.

I heard Obama’s refusing to negotiate with Republicans, is that right?

Yes, at least officially. And certainly, he’s refusing to negotiate over anything that would damage or delay Obamacare. The administration’s argument is that passing a CR or raising the debt limit is not about asking for new money, but simply for funding to cover spending Congress has already agreed to. As such, the CR or the Debt Ceiling should not be used as negotiating tools.

In practice, however, Mr Obama has hinted that he’s prepared to negotiate around the edges, but he is adamant that he will not be held to ransom by Republicans who are risking a US default in order to insist on a list of totally unrealistic policy concessions.

Will the Republicans really go through with this? Don’t they always reach a deal in the end?

Don’t bet on it. It happened in 1995 and 1996, remember. For hardcore Tea Party Republicans in the House of Representatives shutting down the government would be a badge of honour, and would demonstrate their commitment to fiscal responsibility to the people that elected them.

That sounds extreme?

A lot of people think so, including many senior Republicans who, even though they too despise Obamacare and want to rein in government spending, have berated their Tea Party colleagues for being reckless and unreasonable. Ted Cruz, the junior Senator from Texas who was described as a “wacko bird” by Senator John McCain, is the most high-profile target for criticism.

Many older Republicans remember the 1995-1996 shutdowns under then-then speaker Newt Gingrich and the damage it did to the party. They don’t want a repeat, and nor, polls show, do the public, with a majority opposed to forcing a government shutdown. A Gallup survey found only about one-fifth (22pc) of the US public identify with the Tea Party.

So how come there are enough of these Tea Party types to stop Congress passing a CR or raising the Debt Ceiling?

Actually, in terms of pure numbers, there aren’t. The problem is that the Republican speaker of the House of Representatives, John Boehner, has said he will not table a CR unless it has the majority support of his members. The Democrat-controlled Senate has already rejected a CR that defunds Obamacare and sent it back to the House with those provisions stripped out.

Mr Boehner could table the Senate’s so-called “clean CR”, but he would need Democrat voters to pass it, something that might well render his Speakership untenable. He has now decided to attach a lesser anti-Obamacare provision that delays the legislation for a year, to a new CR and return that to the Senate. However Democrats have already promised they won’t pass anything that attacks Obamacare.

Other options could have included be a temporary – say one week – extension to allow more time to negotiate. Or to convince his Tea Party wing to pass the “clean CR” and then pick the fight again over the Debt Ceiling – a move which has more popular political support. But none of these moves offer a solution to core of this stand-off.

What happens if the US government shuts down?

In the case of failing to pass a new CR, about half of government workers get “furloughed”, or put on temporary unpaid leave. Non-essential services like National Parks, passport and driving license renewal services stop. The last shutdowns in 95 and 96 cost more than $1bn. A shutdown in 2013 is expected to cost twice as much.

Breaching the debt ceiling would be much more serious, however, and would have global ramifications since the government would be instantly unable to meet about 40 per cent of its obligations. The entire credibility of the world’s largest economy is at stake. Just the threat of default in 2011 cost about $19 billion in higher borrowing costs over the coming decade.

Sounds absolutely disastrous. Surely common sense will prevail in the end?

You would think so, but Tea Party Republicans believe in their cause and seem determined to have the confrontation as a point of honour. Under those circumstances, nothing can be taken for granted.